In this case, in principle, the expression high voltage should be understood as meaning an operating voltage of more than 1 kV. The preferred voltage range is below 100 kV and mainly relates to apparatuses and installations which carry heavy current and have rated voltages of typically 10 to 50 kV.
The current-carrying capability of apparatuses and installations such as these is limited thermally. Passive cooling elements, such as cooling ribs, are used for the lower rated-current range. However, the options for increasing the rated-current range with the use of known passive cooling elements are very restricted. Active cooling elements (for example air-air heat exchangers with fans) are therefore used for heavier rated currents, such as those which are carried in heavy-current appliances in the form of generator switches.
In addition to active cooling elements such as these, high-power, passive cooling elements have also already been proposed for use in heavy-current switchgear. High-power passive cooling elements such as these include heat pipes. In the case of heat pipes, the heat which is created by current-flow losses in the switch is used to vaporize an agent. The vaporized agent is transported to an external heat exchanger, where it emits the heat losses formed in the switch again by condensation.
Generator switches are generally encapsulated on a single-phase basis and have an inner conductor which is arranged in the encapsulation and is at high-voltage potential. Heat which is formed by current-flow losses on the inner conductor must be dissipated to the surrounding air through the grounded encapsulation. This means that an electrically isolating path must be located between a vaporizer, which is at high-voltage potential, and a condenser, which is kept at ground potential, for the heat pipe, and must be designed appropriately for the required high voltage (for example 150 kV BIL). The vaporizer and condenser are held in a vacuum-tight manner at the two ends of the hollow isolating body.
Since a high-power passive cooling element such as this has no moving parts such as fans, this cooling element can be used to remove heat losses from the encapsulation at low cost and efficiently. In this case, the hollow isolating body carries out a number of functions, in particular that of carrying the agent and that of isolation of the potentials of the vaporizer and condenser. The reliability of a high-power passive cooling element such as this and of a high-voltage installation equipped with a cooling element such as this is ensured only if the isolating tube carries out the abovementioned functions over many years.
DE 22 38 987 A1 describes an isolating tube of the abovementioned type. This isolating tube forms a heat pipe, together with a metal cylinder which is fitted to one of its ends and with a cooling and closure element which is fitted to the opposite end. Heat losses which are produced by the operating current in a switch with little liquid are dissipated from the switch by this heat pipe. The heat losses are in this case absorbed by the heat pipe by vaporization of a liquid, for example of a fluorocarbon, in the metal cylinder which is attached to those parts of the switch which form the heat losses, and is emitted to the surrounding area by condensation of the liquid vapor on the cooling and closure element. The isolating tube may be composed of a glass-fiber-reinforced epoxy resin or the like and is arranged in the interior of a pole tube, which is filled with quenching liquid and is closed by the cooling and closure element. The individual parts of the heat pipe are connected to one another in a vacuum-tight manner by means of suitable seals or by adhesive bonding. A capillary structure which lines the inner wall of the isolating tube allows the heat pipe to be installed horizontally.
A metal-encapsulated switchgear assembly having heat pipes is described in U.S. Pat. No. 3,662,137 A and in U.S. Pat. No. 4,005,297 A. Each heat pipe may be composed of insulating material. The heat pipe then has a container, which is formed from ceramic, for holding an agent, for example a fluorided hydrocarbon, as well as a capillary structure, which is arranged in the container and is composed of glass fibers.
A heat pipe for use in electrical high-voltage circuits is described in G. Yale Eastman: “The Heat Pipe” Scientific American, Volume 218 (May 31, 1968), pages 38 to 46, in particular on page 42. This heat pipe is produced exclusively from insulating material and contains a glass container, in which a capillary structure composed of glass fibers and fluorided hydrocarbon as an agent for the heat pipe are arranged.
EP 242 669 A discloses a heat-absorbing component of a capillary-pumped cooling circuit, in which the capillary material is porous plastic, such as polytetrafluoroethylene, polyethylene or polystyrene.
A cooling element having a hollow isolating body is already known from DE 2 051 150 A. The hollow isolating body is in the form of a hollow post insulator and keeps a switch-pole conductor track which is at high-voltage potential at a distance from the wall of a pole housing which is filled with a switching liquid. A cooler is held on the wall and is connected via two flow lines, which are arranged in the post insulator, to an annular space surrounding the current path. A vaporization means is located in the cooler, in the flow lines and in the annular space. Current which is carried in the current path heats that part of the vaporization agent that is located in the annular space, via the switching liquid. Vapor that is formed in the process is passed via one of the two flow lines into the cooler, in which it condenses, emitting the heat that was absorbed during vaporization. The resultant condensation falls down in the cooler and forces liquid vaporization agent located in the cooler back again into the annular space through the other flow line, in which annular space a new vaporization-condensation cycle is initiated for the liquid vaporization agent that has been supplied.